The present invention relates to a device for obtaining a constant torque characteristic in a control for an induction motor driven by an inverter.
FIG. 7 shows a block diagram of the conventional control device for an induction motor type electric rolling stock. A current fed from a pantograph 1 is inputted through a filter reactor 2 and a filter condensor 3 to a variable-voltage variable-frequency inverter 4 which generates a variable-voltage variable-frequency (hereinafter referred to as VVVF) alternating current. A three-phase alternating current produced by the VVVF inverter 4 is inputted to a tree-phase induction motor 5 to drive it. A current detected by a current detector 6 is inputted to a current operator (or computing element) 7 in which a motor current I.sub.M is calculated.
A comparator 9 compares the motor current I.sub.M with a motor current command I.sub.P produced by a master controller 8 and a deviation .DELTA.I of I.sub.M from I.sub.P is inputted to an amplifier 10 which is turn outputs a slip frequency F.sub.S. On the other hand, the output of a speed detector 11 is inputted to a speed operator 12 to obtain a motor rotation frequency F.sub.R. The slip frequency F.sub.S and the motor rotation frequency F.sub.R are added by an adder 13 to obtain an inverter frequency F.sub.INV. (Upon power running and upon regeneration, F.sub.S and F.sub.R was subtracted from each other.) The inverter frequency F.sub.INV is inputted to a voltage operator 14 which in turn outputs a motor voltage V proportional to the inverter frequency F.sub.INV. When seen from the inverter 4, the motor voltage V is an output voltage command. In actual, a modulation rate is inputted to a PWM modulating portion of the inverter 4. The inverter 4 operates PWM-modulated in accordance with the inverter frequency F.sub.INV and the motor voltage V.
With the above construction, since a voltage to frequcny ratio (V/F) is controlled to be constant and a current is controlled to be constant by increasing or decreasing the slip frequency F.sub.S, a constant torque is obtained. This system is termed an F.sub.S control system.
The F.sub.S control system involves a problem that the torque is decreased in a low frequency region. A cause of the decrease of torque will be hereinafter mentioned in detail.
According to a report presented by the Technical & Research Report (II Part) of Institute of Electrical Engineers of Japan, No. 109, (April 1981), p. 39, the compensation for the voltage drop of a primary impedance which is a cause of the decrease of torque in a low speed region of an induction motor, is made by correcting V/F in a direction in which V/F increases more than a straight line on which V/F is constant.
In the above prior art, on the premise that the detection of the gap flux of an induction motor is difficult, a terminal voltage to frequency (V/F) line or curve is selected which will provide an approximately constant torque. Namely, since it is difficult to measure the torque of the induction motor, it is not possible to determine V/F which makes the torque ideally constant.
For example, when the above method is applied to an electric railcar, no great problem arises even if the torque is not kept strictly constant. However, for example, upon activation on an ascent gradient in the case of a line having a steep gradient, the decrease of torque gives rise to a problem in regard to the running of the electric railcar since a torque in a low speed region is required for activation.
Also, in an electric locomotive using an F.sub.S control, the decrease (or variation) of torque gives a severe influence upon activation, especially, in the case where heavily loaded freight cars are coupled to each other. Namely, in the case where the torque is small, the activation becomes impossible. In the case where proper correction causes the generation of a torque larger than a required or desired torque, a slip occurs resulting in the difficulty of activation.